What does "matchIndex!=-1" do?

I would like to know what "matchIndex!=-1" does.

I know that the bang means "not" so "!=" means not equal.

I also understand that negative one "-1" means false.

So I guess "!=-1" means not false or true?

Put it all together and the above statement would mean that "matchIndex is true."

Let's say that matchIndex is the index of a word found on a page.

So what it's saying is that the index of a word on the page is true.  But compared to what?

That is what I'm confused about.

Here is an example I found on this forum with what I'm talking about in red:

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var text = "Jack be nimble, Jack be quick, Jack jumped over the candlestick."

var searchTerm = "Jack";

var contextLength = 5;

var matchesCounter = 0;

var startIndex = 0;

var matchIndex = text.indexOf(searchTerm,startIndex);

while(matchIndex!=-1) {

    matchesCounter++;

    var context = text.substring(matchIndex-contextLength,matchIndex+searchTerm.length+contextLength);

    console.println("MATCH " + matchesCounter + " (" + matchIndex + "): " + context);

    startIndex = matchIndex+searchTerm.length;

    matchIndex = text.indexOf(searchTerm,startIndex);

}

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Thanks